Blow-by gases are created by pressure leakage passed piston rings and by reciprocating motion of pistons. A draft tube from the crankcase emits blow-by in the form of aerosol and coalescence droplets. Chemically, these blow-by emissions are in the form of oil droplets, carbon soot, and debris from wear for fugitive dust. Physically, typically about 50% of the mass is less than one micrometer.
There are various systems that use open crankcase engines. These include heavy duty trucks and buses; light vehicles; off-road mobile vehicles; off-road equipment (e.g. industrial equipment, generators); and marine engines.
Tailpipe emissions include gases, particulate matter, and aerosols. These exhaust streams include hydrocarbons, carbon monoxide, soot, oxides of nitrogen, nitric oxide, nitrogen dioxide, particulate matter, sulfate, and other material.